1. Field of the Invention
The present invention relates to an electrophotographic process for image formation by repeated steps of the formation of an electrostatic latent image on an electrostatic latent image carrying member, for example a photosensitive member, the wet development of said latent image, the transfer of the thus developed image and the cleaning of said photosensitive member, and more particularly to an electrophotographic process wherein said wet development is achieved by means of an elastic roller.
2. Description of the Prior Art
As an image forming process, there is already known an electrophotographic process in which an electric or electrostatic latent image (hereinafter simply referred to as latent image) is formed on a so-called photosensitive member utilizing a photoconductive substance, is then developed or rendered visible by means of a liquid developer, and the thus developed image is transferred onto a transfer material such as a paper sheet and fixed thereon for example by heating while the photosensitive member is subjected to cleaning for eliminating the remaining liquid developer whereby said photosensitive member is repeatedly utilized in the above-mentioned process.
The development of such latent image can be achieved by a so-called dry development utilizing a toner material consisting of a dry powdered material eventually mixed with solid carrier particles or by a so-called wet development utilizing a liquid developer consisting of toner particles dispersed in a liquid carrier material, the latter being generally employed in the so-called simplified type of electrophotographic copying machine with a relatively slow process speed.
However, it is recently required to achieve a high speed even in such electrophotographic process utilizing wet development. It is at the same time required to provide an elevated image density and a satisfactory image quality without background fog within a limited developing time.
In such electrophotographic process as explained above, the achievement of high-speed process principally depends on the time required for the development step. Stated differently, the acceleration of the entire electrophotographic process can be easily achieved if the development step can be completed within a limited time. However such high-speed process has not been easy to achieve with the conventional wet development methods such as by dipping a latent image carrying surface into a liquid developer or by directing a jet stream of liquid developer onto said latent image carrying surface. More specifically, the conventional wet development methods scarcely reveal practical defects in the developed image at a relatively low process speed generally in the range of 50 to 100 mm/sec., but are almost unable, at a considerably high process speed generally in the range of 200 to 300 mm/sec., to achieve practically acceptable development due to a lowered image density and background fog resulting from the reduced time allowed for the developing step.
In consideration of the above-mentioned drawbacks, there is already proposed, in the Japanese Patent Laid-Open Sho52-40336, a wet development suitable for the high-speed electrophotographic process, in which a liquid-retentive elastic member provided with a liquid-permeating electroconductive flexible surface is utilized as liquid developer supply means and the development of the latent image on the latent image carrying member is conducted in a pressure contact area thereof with said supply means.
Such development method is advantageous in achieving a high-speed electrophotographic process, in comparison with conventionally known wet development utilizing a developer tank or a developer jet, in that the squeeze-supply of the liquid developer and the squeeze-suction of the excessive developer can be achieved simultaneously by elastic deformation of the elastic member maintained under pressure contact. In this method the above-mentioned elastic member is constructed as a rotary member in the form of a roller or an endless belt (hereinafter simply referred to as elastic roller for the purpose of simplicity) which is maintained in rotary motion in contact with the latent image carrying member. The use of such elastic roller in wet development is certainly advantageous in achieving a high-speed electrophotographic process in that, in addition to squeezing out and absorbing of the liquid developer performed substantially simultaneously as explained in the foregoing, a strong electric field is generated between the roller and the latent image due to the electroconductive roller surface and causes rapid displacement of toner particles, thus enabling sufficient development of the latent image for a limited time. It is however not easy to accomplish a high-speed electrophotographic process simply by the use of such elastic roller, and almost impossible when it is combined with the conventionally known technologies.
In the first place a developing roller provided with an electroconductive surface results in an insufficient halftone reproduction because of the excessively strong electric field applied to the latent image as explained in the foregoing, and may often result in white spots in the solid black image area or in undesirable image reversal due to discharge of the electrostatic charge of the latent image to the conductive surface of the roller.
The Japanese Utility Model Publication Sho 53-33560 discloses a roller composed of an electroconductive core member and an electroconductive porous elastic member provided therearound, which however is unable to prevent the above-mentioned drawbacks because of the direct contact of the conductive member with the latent image, and is apt to result in unsatisfactory development or image blur since the surface of the elastic member, when it is brought into pressure contact with the latent image carrying surface, is tightly pressurized thereagainst to hinder the liquid permeation or is displaced in said contact surface.
In consideration of the foregoing, it is essential, in a roller development allowing the realization of a high-speed electrophotographic process, that the electroconductive porous elastic member is not exposed on the surface of the developing roller and that the developing roller is electrically insulating at least at the surface thereof coming into contact with the latent image carrying surface. However the roller with such modification are still insufficient for the practical purpose and tends to result in various drawbacks such as white spots in the solid black image area, undesirable image reversal or extremely deteriorated halftone reproduction for example from a photograph when the developing roller is rendered more electroconductive, or enhanced edge effect, lack of toner in the solid black image area, loss in the image density, elevated background fog or eventually insufficient image transfer when the developing roller is rendered more insulating.